Spheroidal peening particles adhesively bonded to a woven cloth

ABSTRACT

A rotary wheel, in which spheroidal peening particles are affixed to radially extending circumferentially spaced flaps of fibrous sheet material, is used to impart compressive stress to, or aid in shaping, metallic substrates.

United States Patent [191 Winter et a1.

1 1 Dec. 11, 1973 SPHEROIDAL PEENING PARTICLES ADHESIVELY BONDED TO AWOVEN CLOTH Inventors: Phillip M. Winter; Gary A.

Gardner, both of 2501 Hudson Rd., St. Paul, Minn. 55101 Filed: June 25,1970 Appl. No.: 59,850

Related US. Application Data Division of Ser. No. 746,366, July 22,1968, Pat. No.

US. Cl 161/87, 51/298, 161/92,

161/162,161/168,117/33,117/l38.8 N, 117/161 KP Int. Cl B32b 5/16 Fieldof Search 161/87, 162, 168,

References Cited UNITED STATES PATENTS 7/1955 Storrs et al. 51/2982,729,553 1/1956 Price 51/298 2,820,746 1/1958 Keeleric.... 51/2983,102,010 8/1963 Lang .1 51/298 3,079,243 2/1963 Ueltz 51/298 3,316,0724/1967 Voss 51/298 2,369,462 2/1945 Hurst.... 117/16 3,073,734 l/l963Bemmels 161/167 Primary Examiner-William D. Martin AssistantExaminer-Janyce A. Bell Attorney-Kinney, Alexander, Sell, Steldt andDelahunt [57] ABSTRACT A rotary wheel, in which spheroidal peeningparticles are affixed to radially extending circumferentially spacedflaps of fibrous sheet material, is used to impart compressive stressto, or aid in shaping, metallic substrates.

3 Claims, 5 Drawing Figures PATENTED 1 1975 3.778.241

f N VEN TOPS 40 F IG. 4 PHILLIP M. W//\/ TEI? G RY A.G'ARONEP B Mffiwi MA 7' TORNE Y5 SPHEROIDAL PEENING PARTICLES ADHESIVELY BONDED TO A WOVENCLOTH This is a division of application Ser. No. 746,366,

filed July 22, 1968 now U.S. Pat. No. 3,638,464.

BACKGROUND OF THE INVENTION It has long been customary to shot peen toincrease fatigue strength, to relieve tensile stresses that contributeto stress-corrosion cracking, to form and straighten metal parts, etc. Adetailed description of this process and the materials used therein isfound in the ASM Committee Metals Handbook, Volume 2, 8th Division,1964, pages 398-405, and incorporated herein by reference. Prior artshotpeening processes are also described in numerous U.S. patents, e.g.,U.S. Pat. Nos. 2,542,955 and 2,982,007. In conventional shot peening,spheroidal particles of cast steel, cast iron, glass, etc. are blown ormechanically impelled in a high velocity stream against the surface tobe treated. The individual shot particles produce shallow, roundedoverlapping dimples in the surface, stretching it radially from eachpoint of impact and causing cold working and plastic flow. The resultantcompressive stress tends to counteract tensile stresses imparted to thesubstrate by the preceding rolling, bending, abrading, and similarprocesses.

The degree pf peening, which is generally expressed as peeningintensity,is a function of the weight, size, hardness and velocity of the peeningparticles, exposure time, type of substrate, angle of impingement, andvarious other factors. It is conventional to express peening intensityin terms of Almen arc height, according to SAE Test 1442, described insome detail in Military Specification MIL-S-l3165B. In this test, a thinfiat piece of steel is clamped to a solid block and exposed to a blastof shot, which, as previously indicated, tends to stretch the surface,so that the strip will be curved when removed from the block. Teststrips are SAE 1070 cold rolled spring steel uniformly hardened andtempered to a hardness of 44-5O Rockwell C, 3 i 0.015 inch long and0745-0750 inch wide. The strips are one of three thicknesses: A, 0.051inch i 0.001 inch; C, 0.0938 inch i 0.001 inch; and N, 0.031 inch 1*:0.001 inch. The height of arc of the resultant chord in inches isreferred to as the Almen arc height, greater heights indicating greaterpeening intensity for a given test strip thickness.

Effective though it is for many purposes, conventional shot peeningsuffers from disadvantages which drastically limit its usefulness. Forexample, large and expensive equipment is required for rapidly impellingshot toward a surface and collecting, screening and recirculating theshot particles. Equipment of this type is not readily portable, andhence is suitable only for those metal pieces or parts which can bebrought to the shot peen station. It is virtually impossible to shotpeen a part while it still remains attached to another piece ofequipment.

Despite the foregoing drawbacks to the shot peening process, there haspreviously been no effective altemative. It is to this unsolved needthat the present invention is directed.

SUMMARY OF THE INVENTION The present invention provides a novel meansand method for carrying out shot peening procedures. Peening intensitiesachieved are equal to those obtained by any conventional shot peeningoperation, but the customary elaborate and expensive machinery is notrequired. There is no need, for example, to use special impelling,screening, collecting, or recycling equip ment. The novel device, whichis portable, simple, and convenient to operate, can be driven byequipment no more complicated than a conventional electric motor or aflexible shaft connected to such a motor. Large parts can be peened,even while they remain attached to a still larger piece of equipment,permitting the inservice stress-reduction maintenance of aircraft,automobile springs, etc.

The device of this invention comprises an annulus of radially extendingperipherally separated flaps which are united at their radially innerends to a rigid core. Each such flap is a flexible, tough,tear-resistant fibrous support to which shot peening particles arebonded at spaced portions by a strong, tough organic adhesive. In use,the annulus is mounted on a shaft and rotated rapidly while theperiphery is forced against the substrate to be peened. A portion of theflat face of each flap strikes the substrate in turn, thereby causingthe peening particles to perform their normal peening function, butpreventing their normal uncontrolled scattering In practice, it isadvisable first to determine the approximate peen intensity desired,taking into account the type of substrate involved, the particular metalalloy, geometry of the part and stress condition expected in use. Mostaircraft and automotive manufacturers have either developed their ownshot peening specifications or are able to consult pertinent militaryspecifications. Next, it is recommended that an Almen test specimen beadhered to the actual surface to be peened (using double-coated tape orother suitable means) so that peening intensity can be monitored. Theshot peen wheel and operating speed are then chosen so as to achieve theapproximate intensity desired.

Typical applications for the products of this invention include bothinitial and repair stress conditioning of metal parts .to enhancefatigue strength and resistance to stress corrosion cracking. Among theparts which can be treated are torsion bar springs, leaf springs, oilwell drill pipes, shafts and axles, piston pins, crankshafts, landinggear assemblies, connecting rods, wing strut supports etc.Decarburizing, heavy scale removal, stress conditioning before chromeplating and surface finishing to produce attractive matte luster areadditional applications. Wheels of the invention are also well suitedfor peen forming or peen straightening metallic parts. An operator canaccurately bend or bow the shape of a metal component, for example, aheli copter rotor blade, to the desired configuration by applying a shotpeen wheel to the surface. Since no enclosure is necessary, the partremains in full view at all times. Another outstanding advantage of thepresent invention is the ease of varying the type or size of peeningparticles, which is accomplished simply by changing wheels, whereasconventional shot peen systems require complete cleanout and recharge ofthe shot chamber.

It should be pointed out that flap wheels are broadly old, as isevidenced by patents dating well back into the Nineteenth Century.Although such wheels have previously been used for a wide variety ofbuffing, cleaning, polishing, and abrading operations, it is believedthat there has never heretofore existed a flap wheel constructionsuitable for use in shot peening operations.

BRIEF DESCRIPTION OF DRAWING Understanding of the invention will befacilitated by reference to the accompanying drawing, in which:

FIG. 1 is a view of a self-contained rotary peening wheel made inaccordance with the invention;

FIG. 2 is a segment of a flap of the type used in preparing the wheel ofFIG. 1;

FIG. 3 is a cross-section of the article of FIG. 2, segment of a wovenflap which may be used in preparing the wheel of FIG. 1;

FIG. 4 is a segment of a nonwoven flap which may be used in preparingwheels similar to that shown in FIG. 1; and

FIG. 5 is a segment of a composite woven-nonwoven flap which may be usedin preparing wheels similar to that shown in FIG. 1.

DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENTS Although the principlesof this invention can be embodied in such simple devices as a splitrotary shaft which grips a single elongated shot peening strip, it isgenerally preferred to employ a structure with a greater number offlaps. As is particularly shown in FIG. 1, wheel comprises a pluralityof flaps 11, the radially inner ends of which are adhered to core 12,wheel 10 being driven in the direction of the arrow while being forcedagainst substrate 13. The spacing between circ'umferentially adjacentflaps 11 should be great enough to permit a substantial portion of theradially outer portion of each flap 1] to strike substrate 13. Therotary device is so positioned that the spacing between core 12 andsubstrate 13 is significantly less than the radius of flap 11, therebyenhancing the peening action. As the specific examples which follow willshow in detail, flaps 11 may be formed from fibrous sheet material ofvarious types.

FIGS. 2 and 3 illustrate a section of sheet material, suitable forforming flaps, formed from an open mesh fabric 30 woven from warp yarns31 and weft yarns 32, the yarns being spaced so as to definetherebetween open meshes 33. Bonded to yarns 31 and 32 and generallyresting in meshes 33 are peening particles 34, adhesive 35 serving tobond particles 34 in place and also to unify fabric 30.

FIG. 4 illustrates a section of sheet material which comprises anonwoven lofty three-dimensional fibrous structure 40, individual fibers41 being bonded to each other at points where they contact by means ofadhesive 42. Also bonded to spaced portions of fibers 41 are peeningparticles 43. Abrasive products superficially similar to thisconstruction, but lacking the peening ability of the product shown inFIG. 2, are described in U.S. Pat. No. 2,958,593.

When the individual flaps used in making the device of this inventionare formed from lofty nonwoven fibrous sheet material, the diameter ofthe fibers, or filament segments, influences the performance of thefinished product; fibers having a diameter of at least 40 microns, andpreferably in the range of 40-200 microns are preferred. All otherfactors being equal, when 25- denier nylon fibers (55-microne diameter)are used, their ready mobility imparts a greater peening intensity thenwhen 250-denier (l75-microns diameter) fibers are used. On the otherhand, coarse denier fibers are strong, wear resistant, and tend toprovide better anchorage, especially for coarser peening particles.Nylon fibers of 200 denier (160-microns diameter) have been foundespecially suitable for most purposes. To assure formation of lofty opensheet material, the fibers should be crimped.

FIG. 5 shows in cross-section a composite product 50 formed from openmesh woven fabric and lofty open non-woven mat 70, and suitable formaking peening flaps. Fabric 60 is woven from spaced warp yarns 61 andspaced weft yarns 62, defining open meshes. Bonded to yarns 61 and 62and generally resting in the meshes are peening particles 64, adhesive65 serving to bond particles 64 in place and also to unify fabric 60.Bonded to the back surface of woven fabric 60 is lofty open nonwovenfibrous structure 70, made up of individual fibers 71 adhesively bondedto each other at points of contact by adhesive 72. A wheel made withflaps of such composite material is rotated in a direction such thatwoven fabric 60, bearing peening particles 64, contacts the substratefirst.

Each of the three sheet materials illustrated has advantages for use inthe practice of this invention. For example, products of the type shownin FIGS. 2 and 3 are inexpensive and easy to manufacture, long wearing,and impart relatively pure" peening action. Products of the type shownin FIG. 4 are conformable and better able to extend into and uniformlypeen concavities, recesses and depressions. Composite products of thetype shown in FIG. 5 have outstanding fatigue resistance and the peeningintensity which they generate approximates a linear (and hence fairlypredictable) function of rotative speed.

Regardless of the specific construction of flapforming sheet material,the peening particles are substantially spheroidal relatively smoothimpact-resistant inorganic particles having an average diameter in therange of 0.1-2.5 mm. Because peening particles cover a wide spectrum orspecific gravities, it has been found that the quantity of particlespresent can best be expressed in terms of cubic centimeters per squaremeter of backing, effective amounts ranging between 25l,000 cc/mProducts of the type shown in FIGS. 13 and 5 function effectively withrelatively small amounts of peening particles, e.g., as low as 25 cclm-500 cc/cm performing especially well. Products of the type shown inFIG. 4 function effectively with a relatively large range in the amountof peening particles; l00-1,000 cc/m can be used, 400-700 cc/m beingpreferred. Individual peening particles should be spaced sufficientlyfar apart so that their ability to make distinct and separateimpressions on the workpiece is not impaired. In order to insure thatthe peening particles are retained on the flaps, each such particleshould have at least somewhat more than half of its diameter surroundedby bonding adhesive.

Peening particles can be formed from any of the materials used inconventional shot peening operations, e.g., cast steel, cast iron, glassbeads, and the like, or such hard and dense materials as tungstencarbide. Generally speaking, hardness of the peening particles is notespecially critical, provided that it is harder than the substrate.Relatively large particles (e.g., 2.5 mm) are not only difficult toanchor firmly to flaps but also make it difficult to obtain uniformpeening coverage. On the other hand, relatively small particles (e.g.,0.1

mm or less) are easy to anchor, afford uniform peening coverage and arequite effective in imparting residual compressive surface stress, albeitto only a shallow depth. To reduce breakage, friable peening particlesshould be avoided. d

The adhesive which bonds the peening particles to fibers of theindividual flaps should, of course, be both fiberand peeningparticle-adherent. Additionally, it should have a tensile strength of atleast about 150 kg/cm and an ultimate elongation at break of at leastabout 100 percent. Suitable adhesives include various blends of phenolicresin and butadienezacrylonitrile copolymer, and particularly thereaction product of a polytetrahydrofuran prepolymer and a chainextending agent. Suitable adhesives of the latter type include thereaction product of isocyanate-terminated polytetrahydrofuran prepolymerand methylene dianiline, and the reaction product of an amine-terminatedpolytetrahydrofuran prepolymer and a bisphenol A-type epoxy resin.

Further understanding of the invention will,be gained from the followingspecific examples, in which all parts are by weight unless otherwisenoted.

Example 1 This example describes the manufacture of a shot peening flapwheel in which the individual flaps are formed from a nonwoven fibrousweb.

An adhesive was prepared by adding to a tin-plated container 40.0 partsof ethylene glycol monoethylether acetate solvent and 4.02 parts of p,pmethylene dianiline (MDA), a high speed propeller mixer being used tomix the two components until the MDA had dissolved. To the solution wasthen added 31 parts of a 100 percent solids urethane elastomer withblocked isocyanate curing sites, formed by the reaction of polyetherglycol with an excess of aromatic diisocyanate and a ketoxime, having amolecular weight of less than 200 (commercially available from E. 1.duPont de Nemours & Company under the trade designation Adiprene BL-l6). Also added at the same time were 12% parts of a finely divided talcfiller and 1.45 parts of glycidoxypropyl trimethoxy silane (commerciallyavailable from Dow Corning Corporation under the trade designationsilane Z6040"). Although not absolutely necessary, it has been foundthat the silane improves adhesion to both fibers and peening particles,thereby improving wear life of the finished wheel product. All of theingredients were then mixed for an hour, the viscosity of the resultingmixture being approximately 100 cps. at 24 C.

A lofty open nonwoven web was fonned, using a Rando-Webber machine, from200-denier crimp-set lrfi-inch staple nylon fibers, the web having athickness of approximately one-half inch and a weight of approximately31 mgm/cm The web was then pre-bonded by passing it through a pair of70-durometer rubber squeeze rolls, the lower of which was immersed in abath of adhesive described in the preceding paragraph, applying acoating weight (solids basis) of approximately 13 mgm/cm". The web waspassed through a l40l60 C. oven so that the adhesive was tack free, evenwhile warm, the resulting web thickness being on the order of onecentimeter.

An adhesive formulation identical to that described in the secondparagraph of this example was prepared, except that the amount ofethylene glycol monoethyl ether acetate was reduced to 20 parts,resulting in a viscosity of 600 cps. at 24 C. The pre-bonded web wasthen passed through squeeze rolls as in the preceding paragraph, so asto apply the adhesive in an amount equal to approximately mgm/cm (solidsbasis). Size No. 460 cast iron shot peening particles (nominal diameter0.0460 inch, or 1.17 mm), having a Rockwell C hardness of 60-65, wereblown into each face of the adhesive-coated web, thereby assuringuniform penetration throughout, a total of approximately 450 mgm/cm(about 600 cc/m being applied. The make resin was then cured in the samemanner as the prebond. A light sandsize coat (about 30 mgm/cm) of thesame adhesive was applied with a hand controlled paint spray gun and theadhesive cured at approximately 65 C. for 15 minutes and approximately140 C. for 2 hours. The ultimate thickness of the finished productremained approximately one centimeter.

From the cured nonwoven fibrous sheet material referred to in thepreceding paragraph, 38 l /-inch x 2%- inch flaps were cut and stackedin a lr-inch wide steel channel at the bottom of which had been placed astrip of normally tacky and pressure-sensitive adhesive filament tape,adhesive side up. A phenolic resinimpregnated spiral-wound jute corehaving an inner diameter of three inches and an outer diameter of 3-%inches was positioned on top of the center flaps, and the strip of tape,to which one 1 19-inch end of each flap was now firmly adhered, usedtoform the flaps into an annulus about the core. The flaps uniformlyperipherally spaced in radial alignment about the core and a secondstrip of filament tape was then wrapped around the periphery of theresultant annulus and used to tighten the flaps. The core was thenremoved and painted with an epoxy-polyamide resin (equal parts of Epon828, commercially available from Shell Chemical Company and Versamid125, commercially available from General Mills, together with 2 percentblown silica filler, commercially available from Godfrey L. CabotCorporation under the trade designation Cab-O-Sil M5). The radiallyinner ends of the flaps were then painted with the same resin, the corereinserted in the annulus, the resin cured at 65 C. for one-half hourand C. for one-half hour, and the tape then removed. Two other wheels,identical except for incorporation of a lesser number of flaps, wereprepared in the same manner.

Each of the wheels described in the preceding paragraph was mounted on ashaft, driven at about 3,500 rpm (3,750 rpm, no load) and urged againstan Almen A test strip with a force of 13 lbs.., arc height peeningintensity being measured periodically. Results are tabulated below:

Arc height peening intesity, mils, at times noted It appears that thepeening intensity attained after any given time at constant speed andpressure is an inverse function of the number of flaps in the wheel. Itfurther appears that the more densely packed the wheel, the lessvigorous, but the more closely controllable, the peening intensity whichcan be attained. It is presently felt that flat separation of at leastor one-half inch at the tip, whichever is greater, is desirable.

Another wheel similar to those described above, was formed using 13flaps and tested at several speeds for the peening intensity imparted toan Almen A test strip after 60 seconds. Results are tabulated below:

Arc height Speed, rpm peening intensity, mils The wear life propertiesof wheels of the type described in this Example 1 are dependent uponoperating condition, geometry of substrate, number of flaps and size ofshot. A flap wheel containing 38 flaps on a 3-inch core, 8 inch totalwheel diameter, cast iron shot size 280, can be operated on relativelyflat, smooth surfaces for about 40 hours at essentially constant peeningeffectiveness before the flaps are worn down. If size 550 shot isemployed, wheel life is reduced to about 5 hours.

Wheels of this type function well when mounted on an electricgrindingtool driven at a speed which will impart a velocity of 500 to8,000 surface feet per minute to the shot particles at the periphery ofthe wheel. The motor should have sufficient power to maintain a nearlyconstant velocity under load. The hand held tool is applied to thesurface in short back and forth strokes (e.g., on the order of 4inches), so as to cover the entire surface evenly and uniformly.Ultimate control of applied stress uniformity is obtained if automatedsheet metal grinding or finishing equipment is employed; however veryacceptable results are readily achieved by a hand operation. Visualinspection of the pattern of peen impressions provides an effectivemeans for estimating uniformity. Although peening intensity is notseriously affected by variations in applied load pressure at constantspeeds, it continues to increase with time of exposure until thesaturation intensity level is reached.

An extremely simple but particularly versatile and effective peeningtool can be made with a single 1 inch x 1% inch strip of shot peeningmaterial of the type described in Example 1 by gripping the strip,midway between its ends, in a bifurcate mandrel. This tool should bemounted on a high speed electric or air motor grinder chosen to achieveabout 6,000-l0,000 rpm under load. Not only is this type of tool readilycapable of imparting high peen intensities, but it can also beconstructed in very small dimensions to facilitate the peening of innersurfaces of holes, pipes, recesses and the like. Such simple devices arealso extremely useful for rapidly evaluating a variety of peening webconstructions. The table set forth below shows the peening intensitiesattained in one minute at 10,000 rpm on Almen A test strips, utilizing avariety of peening particle sizes and types:

Nominal particle diameter, mils Arc height peening intensity, mils,

for type of peening particle Example 2 This example describes themanufacture of a shot peening flap wheel from peening material similarto that in Example 1 but in which the flaps are formed in a differentmanner.

Nonwoven lofty web material having peening particles bonded throughoutwas prepared as in Example 1, using Size No. 330 cast iron shot (nominaldiameter, 0.0331 inch, or 0.84 mm) and omitting the sandsize adhesive.Two 5 inch diameter discs, each having a 1% inch center hole, were thendied from the web. From the periphery of one disc, 14 equally spacedradially extending notches, about one-half inch wide at the peripheryand 1 inch deep along the radius, were then cut. The resultant producthad the general appearance of a spur gear, with 14 teeth, each aboutone-half inch wide, 1 inch long and inch thick, and was analogous to theflap wheels made in Example 1.

Each of the two discs described in the preceding paragraph, one havingteeth and the other not, was then mounted between 3-inch side plates,installed on a flexible shaft machine, and applied to a series of AlmenA test strips for 60 seconds with a force of 7 lbs. at a range ofspeeds. Results are tabulated below:

Arc height peening Temperature of Almen A It is noted that peeningintensity is directly related to the speed at which peening particlesstrike the test strip. The foregoing table also shows that a toothedwheel peens more effectively and at lower temperature than a continuouswheel, in which the binder tended to smear on the test strip at highspeeds. Continued use at high speeds caused some tooth loss, indicatingthat this method of flap formation is less desirable than that describedin Example 1. It is considered desirable, however, for the fibers of theweb to wear away when peening particles are lost during use, permittingfresh particles to assume the peening role.

Example 3 This example describes the manufacture of a shot peening flapwheel in which the backing is a square weave open meash cloth.

To a tin-plated steel container was added 13 parts of methyl ethylketone and 13 parts of MDA, as described in the preceding example, apropeller mixer being used to dissolve the MDA in the solvent. Nextparts of Adipreno BL-16" and 4.7 parts of Silane Z6040" (see previousexample) were added and all components mixed for 30 minutes, theresulting viscosity being 2,000 cps. at 24 C.

A square weave greige cloth nylon scrim formed of sixteen 840-denierthreads (140 filaments per thread) per inch in both the warp and weftdirections, having a 250 lb/in. (approximately 45 kg/cm) grab tensilestrength in both length and cross directions, sized with a vinyl resinin an amount equal to about one-fourth the fiber weight, and weighingapproximately 16 mgm/cm, was obtained. To one face of the cloth justdescribed was applied the resin described in the preceding paragraph,the coating weight being on the order of 17 mgm/cm' (dry basis). SizeNo. 390 (nominal diameter 39.4 mils, or about 1 mm) cast iron shot wasthen sprinkled on the adhesive coated face of the cloth to provide arandom coating of approximately 0.14 gm/cm an amount equal toapproximately 180 cc/m The resin was then cured for minutes atapproximately 65 C. and for minutes at approximately 140 C., after whicha sandsize coat of the same resin was applied in approximately the sameamount as the previous coat. The resin was then cured for 15 minutes atapproximately 65 C. and for 2 hours at approximately 140 C. The castiron peening particles tended to collect in mesh openings, where theywere nestledin a resin socket.

The sheet material described in the preceding paragraph was then cutinto individual l /-inch X l-%-inch flaps. ln the peripheral exterior ofa core of the type described in Example 1, l2 equally spaced slots threethirty-seconds-inch wide and three thirty-seconds'inch deep were cut soas to extend parallel to the axis of the core. Each such slot was thenfilled with an epoxypolyamide resin of the type described in Example 1and the core placed on its side on a silicone-coated release paper. Intoeach slot was then inserted a flap, the surface of the flap bearing thepeening particles facing the same direction in each case. The resin wasthen cured at room temperature (24 C.) for 12 hours.

In general, the desiderata relating to the type of power tool to employ,the effects of speed, size of shot, type of shot, dwell time, etc.,which were discussed for the construction described in Example 1 alsoapply generally to the construction of Example 3. The notabledifferences are that the wheel of Example 3 is capable of generatinghigher peen intensities and exhibits a lower rate of wheel wear comparedwith the wheel of Example 1, all other conditions being equal. Forexample, roughly 30 percent increase in peen intensity and four folddecrease in wheel wear rate are observed for the wheel of Example 3 ascompared with Example I. It should be noted however, that the attractivefeature of lower wheel wear for the wheel of Example 3 is somewhatoffset by the high peen intensities obtained in short times, which maycause difficulty in controlling and maintaining uniformity.

Example 4 Mobay Chemical Company under the trade designation MultronR-68) 100.0

percent solids solution of the reaction product of one mol oftrimethylolpropane and 3 mols of toluene diisocyanate in ethyl acetate(commercially available from Mobay Chemical Company under the tradedesignation Mondur CB-75) 83.4

Hexanetriol 5.04

Ethylene glycol monoethyl ether acetate 150 The composition was thencured by heating 15 minutes at 65 C. and 45 minutes at C.

The woven peening sheet material described in Example 3 was coated onthe back (side opposite the peening particles) with a thin layer of themake resin described in Example 3, firmly forced against one surface ofthe nonwoven web described in the preceding paragraph, and the resincured by heating as in Example 3. From the cured laminate l2 1% inch X2% inch flaps were formed and adhered to the periphery of a l-% O.D.core similar in construction to that used in Example 1, all flaps beingaligned. to face in the same direction. The stiffness and resilience ofeach flap were significantly greater than for the flaps of Example 3,facilitating both handling and wheel formation. The flaps extended insubstantially straight radial array, even after extended use, ratherthan assuming a curved configuration.

The wheel of this Example 4 could be used for the same purposes as thewheel of Example 3, but permitted a more precise control of the peeningprocess. When the wheel was examined after extended use, it was foundthat the individual flaps remained in substantially straight radialarray, rather than assuming a curved configuration. It was also foundthat the flaps did not fatigue at their point of attachment to the core.The nonwoven lamina provides sufiicient strength that a wide variety ofopen mesh scrim cloths may be used to achieve a desired particlespacing, even though such cloths may not be especially strong.

Wheels of the type described in this Example 4 have proved highlyeffective in the contour adjustment of helicopter rotor blades. Suchblades, which are typically about 1% feet wide and 22 feet long, aremade from a relatively thin aluminum skin laminated over a honeycombcore to a precisely contoured airfoil shape. Each blade may cost severalthousand dollars, yet it will be scrapped for any deviation from a 0.015inch or less contour range at several check points. The wheel of thisexample can be applied to the appropriate side of an out-of-toleranceblade to stretch the skin and thus adjust contour as much as i 0.050inch.

In some operations, it is desirable to utilize a process in which shotpeening is combined with a certain amount of abrasive cleaning, e.g.,.in the removal of scale from hot rolled steeLThe inclusion of at least aminor amount of abrasive particles in structures of the type disclosedhereinabove permits a combination action in which the scale is shockedloose by the peening particles and whisked away by the abrasiveparticles, the peening particles insuring that the ultimate stressimparted to the work surface is compressive, rather than tensile, andhence that the peak strength of the product remains improved. Productsof this type can also be made by alternating peening particle-coatedflaps and particlecoated particle-coated flaps, the specific ratio ofone type of flap to the other being governed by the result which it issought to achieve.

As has previously been indicated, shot peening improves the fatigueresistance of metals. A commonly used device for evaluating fatigueresistance is the R. R. Moore High Speed Fatigue Testing Machine, arotating beam type machine in which a notched cylindrical metal testspecimen functions as a simple beam symmetrically loaded at two points.When the specimen is at rest, fibers above its neutral axis are incompression and those below this axis are in tension. As the specimenrotates, the stresses in these fibers are gradually reversed until atthe end of one-half revolution those originally in compression are intension. Thus, during each complete revolution the test specimen passesthrough a complete cycle of flexural stress; the nominal operating speedproduces 10,000 cycles of stress per minute at a bending moment whichcan be adjusted to a value which will yield the desired extreme fiberstress for the test specimen. Detailed information is available from TheWarner & Swasey Co. or Baldwin-Lima Hamilton Corporation.

To determine the effectiveness of various flap wheels made with Size 330cast iron peening particles in improving fatigue resistance, a series oftests were run on the R. R. Moore Machine with 7075-T6. Aluminum testspecimens each having a 0.55 inch diameter circular cross section andgrooved with a 0.125 inch radius notch, treating them for one minute ata speed yielding a saturation peening intensity of Almen A 0.008 inch.Results are tabulated below:

Fatigue life, cycles I, at

extreme fiber stress (psi X indicated Type flap 25 50 Control-untreated140 20 2.7 0.9 Nonwoven (similar to Example 2) 6000 360 4] 7.7

Woven (similar to Example 3) 3900 I 58 7.9 Wovennonwoven laminate(similar to Example 4) 4000 600 7.2

The results indicate that peening with wheels of this invention improvesfatigue life by a conservative factor of from about 8 to 35, with thegreatest improvement being shown at lower stress values. The resultsalso show that peening with the wheels of this invention permits a givenpart to be subjected to a higher stress level than one which is notpeened, while maintaining the same useful life.

We claim: 1. As a new article of manufacture, having particular utilityfor fabricating flaps for use in a self-contained portable rotarypeening device having an annulus of said flaps united at their innerends to a rigid core,

an open-mesh woven cloth formed of nylon yarns of about 800 denier andhaving a grab tensile of about 45 kg/cm;

relatively smooth, impact-resistant hard inorganic spheroidal peeningparticles having average diameters in the range 0.1 2.5 mm dispersedover one face of said cloth in an amount equal to about 500 cc/m ofcloth; and

a tough strong elastic adhesive, having an ultimate elongation of atleast 100 percent, bonding together the warp and weft fibers of saidcloth and also bonding said particles to said fibers.

2. The article of claim 1 wherein the adhesive is a chain-extendedpolymer of polytetrahydrofuran.

3. The article of claim 1 wherein the adhesive is a blend of phenolicresin and rubbery butadiene: acrylo- A UNITED STATES PATENT OFFICECERTIFICATE OF CORRECTION "Patent No. 3,778,211 Dated 1973 fi w PhillipM. Winter and Gary A. Gardner It is certified that error appears intheabove-identified patent and that said Letters Patent are herebycorrected as shown below:

"[73] Assignee: Minnesota Mining and t Manufacturing Company, St. Paul,

Minn."

Should be inserted between [75] and [22] Col. 3, line 66, "microne"should be "microns" Col. A, line lO, "or" should be "of" Col. 6, line56, intesity" should be "intensity" A Col. 8, line H6; at lower" shouldbe "at a lower" Col. 8, line 65, "Adipreno" should be "Adiprene" Col.10, line 65, after and, "particlecoated" should be A "abrasive" Signedand sealed this 23rd day of July 197 (SEAL) jAttest:

McCOY M. GIBSON, JR; C. MARSHALL DANN Attesting Officer Commissioner ofPatents

2. The article of claim 1 wherein the adhesive is a chain-extendedpolymer of polytetrahydrofuran.
 3. The article of claim 1 wherein theadhesive is a blend of phenolic resin and rubbery butadiene:acrylonitrile copolymer.